A developer carrying device is used in an image forming apparatus. The developer carrying device carries a developer containing a toner and a magnetic carrier. The developer carrying device includes an agitating and carrying member that carries the developer in an axial direction while agitating the developer according to the rotation of the agitating and carrying member. The developer carrying device also includes a toner-concentration detecting unit that detects a toner concentration of the developer carried by the agitating and carrying member.
The agitating and conveying member, which is generally a screw member, carries the developer to an area opposed to a latent image bearing member according to the surface movement of a developer bearing member, which is generally a sleeve, while bearing the developer on the surface of the developer bearing member. The developing device transfers the toner in the developer onto a latent image on the latent image bearing member to develop the latent image and obtain a toner image. The developer that has contributed to the development is returned onto the agitating and conveying member in the developing device according to the movement of the developer bearing member. The toner concentration of the developer is detected by the toner-concentration detecting unit while the developer is carried by the agitating and carrying member. The developer is replenished with an appropriate amount of the toner based on a result of the detection and supplied to the developer carrying member again.
Sometimes the volume of the toner in the developer changes due to environmental fluctuation or fluctuation in an amount of electric charge on the toner. In this situation, although the toner concentration has not changed, the conventional toner-concentration detecting unit erroneously detects a change in the concentration of the toner. Such misdetection can be prevented by pressing the developer strongly in a position of detection by the toner-concentration detecting unit to adjust the amount of the toner that affects the toner concentration. For example, Japanese Patent Application Laid-Open No. 6-308833 discloses (see FIG. 10) a graph indicating that a result of detection by a permeability sensor as a toner-concentration detecting unit can be fixed regardless of an amount of charge of a toner by pressing a developer with a force equal to or larger than 30 [g/cm2] (9.8×300N/cm2).